Watermark information (denoted WM) consists of several data symbols which are embedded continuously in carrier content, for instance in (encoded) audio or video signals. The watermarking serves e.g. for identifying the author of the signals. At decoder site the WM is regained by using correlation of the received signal with a known bit sequence, e.g. an m-sequence if spread-spectrum is used as underlying technology.
Most WM technologies transmit redundancy bits for error correction. But such error correction has a limited capacity only. An error correction can correct some symbols, if one or more data symbols cannot be directly recovered at receiver side. But if the capacity of the error correction is exceeded, the WM can not be recovered.
A correlation can be performed by RFFT transforming (one-dimensional real-symmetric fast Fourier transform) two input signal vectors, which each consist of a section of N values of the input signal, to which section N−1 zeroes are attached (i.e. padding by N−1 zeroes), and by multiplying one of the transformed vectors by the conjugated version of the other transformed vector, followed by inverse RFFT transform of that frequency domain product vector.
Invention
Spread-spectrum watermarked signals are decoded using a correlation or a matched filter. If (in spite of error correction) the level of a spread-spectrum watermark is too low or if too much noise had been added during the transmission of the audio or video signals, the correlation does not show a clear peak, which means that the WM information bits cannot be recovered.
A problem to be solved by the invention is to improve the unambiguousness of the correlation result. This problem is solved by the method disclosed in claim 1. An apparatus that utilises this method is disclosed in claim 2.
The invention solves this problem by adaptively pre-filtering the correlation result or vector in the frequency domain before inverse RFFT transforming it. The correlation result is improved by removing in the frequency domain a number of frequency bins of that vector which frequency bins do not have a positive impact on the correlation peak at location zero, i.e. which frequency bins do not contribute to that peak. Thereby the noise level in the correlation or matched filter output is reduced and the watermark information items can be recovered or decoded correctly from the received watermarked audio or video signal. The inventive pre-filtering is carried out following synchronisation of the decoder, i.e. the filtering has no influence on the decoder or receiver synchronisation.
The invention improves the reliability of the watermark signal decoding or recovering and makes watermarking of critical sound or video signals much more robust, which may make the difference between regaining WM information and regaining no WM information at all.
The invention can be used in all watermarking applications in which a correlation or matched filter function are used and in which the watermark signal detector and the received audio/video signal are already synchronised.
In principle, the inventive method is suited for decoding watermark data information from a watermarked audio or video signal on which said watermark data information was modulated, using correlation with a pre-determined data sequence, said method including the steps:                receiving a section of said watermarked audio or video signal;        correlating said section with at least one pseudo-random sequence;        deriving from the correlation result or results, respectively, said watermark data information,wherein said correlations are carried out in the frequency domain wherein bins of the resulting spectrum or spectra for said section are set to zero which do not have an impact on forming a peak at location zero in said correlation result or results, followed by inversely transforming the such processed spectrum or spectra into the time domain.        
In principle the inventive apparatus is suited for decoding watermark data information from a watermarked audio or video signal on which said watermark data information was modulated, using correlation with a pre-determined data sequence, said apparatus including:                means being adapted for receiving a section of said watermarked audio or video signal;        means being adapted for correlating said section with at least one pseudo-random sequence and for deriving from the correlation result or results, respectively, said watermark data information,wherein said correlations in said correlation means are carried out in the frequency domain wherein bins of the resulting spectrum or spectra for said section are set to zero which do not have an impact on forming a peak at location zero in said correlation result or results, and wherein said correlation means inversely transform the such processed spectrum or spectra into the time domain.        
Said correlation can be performed using a one-dimensional real-symmetric fast Fourier transform in which, to said section representing a first transformed vector and having N values and to said pseudo-random sequence representing a second transformed vector having N values, N−1 zeroes are attached, and wherein one of said transformed vectors is multiplied by the conjugated version of the other transformed vector in order to form said correlation in the frequency domain, and wherein said inverse transform is an inverse one-dimensional real-symmetric fast Fourier transform.
Advantageous additional embodiments of the invention are disclosed in the respective dependent claims.